Sun exposure as a key factor influencing saproxylic beetle communities

The ability to locate and colonize ephemeral deadwood resources is crucial
to saproxylic beetle assemblages. Saproxylic beetles locate suitable
substrates mainly through visual cues and via olfactory cues emitted by
deadwood, other insects and, wood-decaying fungi. For the conservation of
saproxylic beetles, it is essential to understand which abiotic factors
and biotic interactions most significantly influence their habitat
requirements when locating suitable substrates. In a field
experiment, in sunny and shaded plots, we exposed 400 bundles of freshly
cut deadwood, each consisting of three logs with a combination of
different tree species and treatments (i.e., fungi inoculation), mimicking
biotic interactions. We sampled arriving saproxylic beetles via sticky
traps directly applied on the deadwood substrate to evaluate the effect of
sun exposure and the biotic interactions on colonizing saproxylic
beetles. We found higher species numbers and an abundance of
saproxylic beetles under shaded than sunny conditions, but there was no
effect on the standardized number of species (diversity). However, we
observed a shift in species diversity from sunny to shaded conditions from
early to late season. Beetle assemblages differ between shaded and
sun-exposed deadwood. Treatments (fungi inoculation, sterilization, and
burning) did not affect saproxylic beetle colonization.  Our
results suggest that colonization of freshly cut deadwood by saproxylic
beetles is mainly driven by sun exposure rather than biotic interactions,
despite assumed close associations between beetles and fungi. To protect
the full spectrum of saproxylic beetle species, we recommend managing both
sunny and shaded areas, as each provides unique habitats that support
different species compositions.